1. Field of the Invention
The present invention relates to a method of exposure and a projection type exposure apparatus for forming a fine pattern for semiconductor integrated circuits, liquid crystal devices, or the like.
2. Related Background Art
Formation of circuit pattern for semiconductor usually requires a process of the so-called photolithography. This process employs a method of copying a reticle (mask) pattern on a substrate such as a semiconductor wafer. The substrate is coated with a photosensitive photoresist, and a circuit pattern is copied on the photoresist according to an illumination light image, that is, a pattern of transparent portion in the reticle pattern. In a projection type exposure apparatus (for example a stepper), an image of circuit pattern drawn on the reticle is projected through a projection optical system onto the substrate (wafer) to form an image thereon.
There is an optical integrator (such as a fly eye type integrator, a rod type integrator, and an optical fiber) employed in an illumination optical system for illuminating the reticle to make uniform an intensity distribution of illumination light irradiating the reticle. The fly eye type integrator (fly eye lens) is a group of lenses, in which several ten's unit lens elements having the same shape are arranged in a plane normal to an optical axis of the illumination optical system. When the intensity distribution is made uniform using the fly eye lens, there are a Fourier transform relation between a reticle side focal plane (exit side focal plane) and a reticle surface (pattern surface) and another Fourier transform relation between the reticle side focal plane and a light source side focal plane (entrance side focal plane). Accordingly, an imaging relation (conjugate relation) is established between the pattern surface of the reticle and the light source side focal plane of the fly eye lens (more precisely each light source side focal plane of unit lens element in the fly eye lens). Thus, the illumination light beams from the respective lens elements (secondary light source images) in the fly eye lens are added (superimposed) through a condenser lens on the reticle to be averaged, whereby good illuminance evenness may be attained on the reticle.
Incidentally, the reticle side focal plane (exit plane) of the fly eye lens is an optical Fourier transform plane to the pattern surface of the reticle, and each position of lens element in the exit plane corresponds to an incident angle (more precisely a sine of the angle) of an illumination beam from said each element into the pattern surface of the reticle. Thus, if an aperture stop (.sigma. stop) is located near the exit plane, a range of incident angle of illumination light into the reticle pattern would be limited. An aperture shape of the aperture stop was conventionally a circular transparent portion with center on the optical axis. A coherence factor (.sigma. value) is generally defined as a ratio of incident angle range of illumination light into the reticle pattern, which is determined by a radius of the circular aperture, to reticle side numerical aperture of the projection optical system.
Recent attention has been drawn to the annular zone illumination method (as described in Japanese Laid-open Patent Application No. 61-91662) and to the modified light source method (as presented at the scientific lecture in Fall 1991, Society of Applied Physics, Japan) for improving the resolution and the focal depth. In the annular zone illumination method, a stop having an annular transparent portion (as will be referred to an annular zone stop) is located near the optical Fourier transform plane to the reticle pattern in the illumination optical system, for example, near the exit plane of the fly eye lens, to shield illumination beams near the optical axis of the illumination optical system. In the modified light source method, a stop having at least one transparent portion eccentric to the optical axis of the illumination optical system (as will be referred to as a modified light source stop) is located near the exit plane of the fly eye lens to illuminate the reticle pattern with illumination beams being inclined .thereto. Inventors filed patent applications, for example, U. S. Serial No. 791,138 (Nov. 13, 1991) and U. S. Ser. No. 847,030 (Apr. 15, 1992) to disclose the modified light source method.
The fly eye lens is composed of a plurality of square or rectangular lens elements arranged in matrix. The projection type exposure apparatus normally employs the Kohler illumination, in which images of a light source such as a mercury lamp are formed on the exit plane of the fly eye lens. Thus, the secondary light source on the exit plane of the fly eye lens is composed of discrete light source images each being concentrated around the central area of each lens element, which means that the entire exit plane is not emitting uniform light. If the .sigma. stop is provided for such a discrete secondary light source, some of images of secondary light source overlap an edge of the stop (a boundary between a light transmitting portion and a light shielding portion), which will be an unstable factor such as a dispersion of illumination light amount. A method effective to eliminate such unstable factor is for example what is disclosed in U.S. Pat. No. 4,939,630, in which an aperture shape of the .sigma. stop follows an arrangement of lens elements constituting the fly eye lens. That is, the boundary between the light transmitting portion and the light shielding portion is substantially coincident with a border line between the elements. The aperture shape may be also determined to follow the arrangement of lens elements in each of the annular zone stop and the modified light source stop. Further, U.S. Pat. No. 4,939,630 discloses so-called double fly eye lenses, which are two sets of fly eye lens arranged in series.
A typical shape of pattern area (area illuminated with illumination light) on the reticle is rectangular, matching with the shape of semiconductor chip to be formed on the wafer. The respective lens elements in the fly eye lens thus usually have a rectangular cross section. This inevitably makes an arrangement pitch on the shorter side of rectangle (lens element) different from that on the longer side in the fly eye lens. This in turn results in formation of discrete images of secondary light source (or tertiary light source in double fly eye lenses) on the exit plane of the fly eye lens at different longer side and shorter side pitches.
No study is made in U.S. Pat. No. 4,939,630 as to optimization of aperture shape of .sigma. stop in relation with the arrangement of lens elements (light source images). Nothing was studied about optimization of aperture shape in relation to the arrangement of lens elements, either, in the annular zone stop disclosed in Japanese Laid-open Patent Application No. 61-91662, or, in the modified light source stop as presented at the '91 Fall scientific lecture, Society of Applied Physics, Japan. Therefore, there has been such a problem that two types of pattern extending perpendicular to each other (in the directions of the longer side and the shorter side of the lens elements), which will be referred to as a vertical pattern and a horizontal pattern, would form respective replication images in line widths (light amount distributions) different from each other. A further problem was found in the modified light source stop that a focal depth of the vertical pattern is greatly different from that of the horizontal pattern.